Slide antennuator

ABSTRACT

Apparatus for effecting a smooth, quiet, and reversible operation without backlash of the rotatable control member of an electrical device which member is rotatable to vary the electrical output characteristic of such device. A substantially rigid rotatable cam of the form of a helically twisted ribbon presenting opposed surface portions extending helically lengthwise of the axis of rotation of the cam, a resilient coupling between the cam and control member, cam drive means comprising a roller carriage slideable with low friction along a linear path determined by a fixed slide rod and roller means carried by the carriage spring biased into resilient contact with said opposed surface portions of the cam.

1111a States Patent 1191 Bloom et al. June 5, 1973 54 SLIDE ANTENNUATOR 2,379,328 6/1945 Weingart ..74/89 [75] Inventors: Robert Bloom Lathrup village; 2,311,423 2 1943 Wem ..74/569 Assignee:

Filed:

Appl. No.: 182,660

Charles Sturtz, Romulus;

Timothy P. McGrath, Detroit, all of Mich.

Audio Designs and Manufacturing, Inc., Roseville, Mich.

Sept. 22, 1971 References Cited UNITED STATES PATENTS Thorne Nutting ..74/89 .l5 ..74/57 Lippitt ..74/57 /4 1 al/e1 W I! Primary ExaminerCharles J Myhre Assistant Examiner-Wesley S. Ratliff, Jr. Attorney-Sidney A. Ochs [57] ABSTRACT Apparatus for effecting a smooth, quiet, and reversible operation without backlash of the rotatable control member of an electrical device which member is rotatable to vary the electrical output characteristic of such device. A substantially rigid rotatable cam of the form of a helically twisted ribbon presenting opposed surface portions extending helically lengthwise of the axis of rotation of the cam, a resilient coupling between the cam and control member, cam drive means comprising a roller carriage slideable with low friction along a linear path determined by a fixed slide rod and roller means carried by the carriage spring biased into resilient contact with said opposed surface portions of the cam.

10 Claims, 1 1 Drawing Figures 32 /////A I W suns ANTENNUATOR BACKGROUND OF THE INVENTION This invention relates to apparatus for operating devices for instance, electricial devices, having a control member rotatable for varying the output characteristic of the device and especially concerns apparatus for operating the control member of a potentiometer of an attenuator for electronic sound circuits, for varying the electrical 18-gauge output of the potentiometer and hence the effect of the attenuator.

An object of the invention is to provide apparatus of the above character capable of utilizing substantial linear movements to effect small changes in output and register such changes on a readily visible scale.

Another object is to provide mechanism as in the previous object capable of effecting reversible rotation of the control member and hence output changes in the device with a minimum of mechanical and electrical noise and without backlash or other lost motion in the operating mechanism likely to cause inaccurate readings.

A specific object is to provide means including a roller slide mechanism capable of effecting the preceding objects.

A further specific object is to provide operating mechanism for the control member of the apparatus of the preceding objects comprising a helical cam resiliently coupled to the control member, and slide means operable over a guided linear path and carrying roller elements in resilient drive relation with the cam for rotating the cam in response to operation of the slide means.

The above and other objects, features, and advantages attending the present invention will be understood from a consideration of the description of a preferred embodiment of the invention following the accompanying DESCRIPTION OF THE DRAWINGS Wherein:

FIG. 1 is a front elevational view of the indicia means of the apparatus of the invention showing the manually operable slide positioning knob and pointer and scale of values in decibels registered at different positions of the knob;

FIG. 2 is a side elevational view with parts broken away to show a section taken generally at 2-2 of FIG.

FIG. 3 is a side elevational view of a part of the apparatus of the invention looking in the direction of the arrows 3-3 in FIG. 2 and showing the output terminal block;

FIG. 4 is a sectional view taken at 4-4 of FIG. 2 showing the roller slide mechanism of the apparatus of the invention;

FIG. 5 is a sectional view taken at 5-5 of FIG. 2 showing the manner of resiliently coupling the cam and potentiometer control member of the apparatus of the invention;

FIG. 6 is a sectional view taken at 6-6 of FIG. 2 showing the pivot structure for the cam element of the apparatus of the invention;

FIG. 7 is a schematic electrical diagram of the potentiometer and terminal block of the apparatus of the invention;

FIG. 8 is a sectional view similar to that of FIG. 4, showing a modification using a four roller drive of the cam;

FIG. 9 is a perspective view of a modification of the apparatus of the invention altering the manner of slideably mounting the roller carriage on its supporting rod; and

FIGS. 10 and 11 are end and side elevational views respectively of a further modification of such roller carriage mounting.

Referring to the drawings, in particular to FIGS. 1 to 7 showing a preferred form, the attenuator apparatus of the invention comprises an elongated casing or container generally referred to by the numeral 10, an attenuator device or means referred to by the numeral 10, an attenuator device or means referred to by the numeral 12, cam means generally referred to by the numeral 14 for operating the attenuator device or means, cam drive means generally referred to by the numeral 16 for actuating the cam means 12 and indicia means generally referred to by the numeral 18 on which to visually register the output characteristic or value of the attenuator means 12.

The casing 10 is preferably made of metal and preferably comprises an elongated U-shaped section 20, providing the bottom or rear wall portion 22 and opposite side wall portions 24, 26 of the casing and a pair of end plates 28, 30 abutting the opposite ends respectively of the section 20 providing the end wall portions of the casing. A face plate 32 abuts the forward edges of the section 10 and of the plates 28, 30 to close the open side of the casing and provides the frontal wall of the apparatus and means of mounting the same.

The cam means 12 comprises a substantially rigid rotatable cam 34, preferably in the form of a relatively thin, helically twisted metallic band, or ribbon-like strip 35 whose lower end 36 is pivotally mounted as at 37 in the lower end plate 30 of the casing and whose upper end is flexibly coupled for rotation by a flexible coupling generally referred to by the numeral 39 to the control shaft 40 of the attenuator device 12. The cam 34 provides a cam surface portion or portions, which on the strip 35 are the opposite surface portions 42, 44 extending helically lengthwise of the axis of rotation of the cam adjacent the outer lateral edges 46, 48 of the strip for a purpose to be described. It will be seenfrom FIGS. 2 to 6 that the strip 35 is flat at its opposite ends 36, 38 in the areas of the pivot connection and coupling, these flat portions lying in a plane passing through the axis of the cam. It will be evident too that the helix portion lies between these flats and is essentially that of a double left hand thread having a pitch that is substantially the full length of the cam. In one form of the invention such as shown the cam was made from an l8-gauge Krinch wide stainless steel strip, 5 inches long with 54-inch wide end flats and a 3115? right hand twist between the flats. As evident from FIG. 2 and 4 a transverse section taken through the strip at any point along its length is substantially straight and the mid-points of such sections lie on the axis of the cam.

The pivotal mounting 37 comprises as seen in FIG. 2 and 6, a pin 50 received in a bore 52 in the plate 30. The pin 50 is slightly greater in diameter than the thickness of the cam strip 35 and is fitted in a slot 54 formed by the tabs 56, 58 in the flat portion of the end 36 of the strip 35, and clamped in this position by clamping bars 60, 62 through the use of screws 64, 66. The screw 64 passes through the bar 60 and tab 56 and is threadedly received in the bar 62 whereas the screw 66 passes through the bar 62 and tab 58 and is threadedly received in the bar 60. It will beapparent that the pin"50 as so mounted is central of the cam strip in width and thickness and provides a low friction rotatable shaft.

The flexible coupling 39 comprises as seen in FIG. 2 and 5, a cross bar 70 secured against a flat 72 on the control shaft 40 by a screw 74 threadedly received in the shaft 40 and which bar 70 is spring biased by compression springs 76, 78 against spaced apart tabs 80, 82 formed in the upper end 38 of the strip 35. The springs 76, 78 are carried by screws 84 whose threaded shanks pass through the springs and through suitable openings in the bar 70 and tabs 80, 82 and are threadedly received in nuts 86 which are adjustable to alter the compression of the springs. The fiat 72 is so formed on the control shaft 40 that the axes of the control shaft and cam 34 coincide. The coupling compensates for any misalignment in the axes of the control shaft 40 and cam 34 and also provides a no backlash connection between control shaft and cam.

The attenuator means 12 may comprise an electrical unit or device 88 and associated terminal block 89 both supported by the end plate 28. The device shown is a rotary type potentiometer devicehaving its rotary control shaft 40 coupled as described above to the cam 34. The device 88 has a stationary shell 90 including a threaded extension 92 surrounding the shaft 40 whereby it is secured as by a nut 94 to the inwardly extending leg portion 96 of the casing end plate 28 the leg 96 having an opening 93 through which the extension 92 projects for this purpose. The terminal block 89 is of the five prong or contact type and is secured to the plate 28 by a nut 99.

A potentiometer device such as the device 88 is made by the Allen Bradley Company under the identifying number GO7GO32F252 MA having a so called three terminal bilateral circuit. It comprises a pair of carbon resistance sections or pots 100, 102 shown schematically in FIG. 7. One of these connects with the input terminals Lo and Hi and comprises in parallel a fixed 820 ohm resistance K and a 2,500 ohm variable resistance V. The other section connects with output terminals Lo and Hi and similarly comprises in parallel a fixed 820 ohm resistance K and a 2,500 ohm variable resistance V. The terminals Lo and Lo are interconnected and a carbon wiper W operated by the control shaft 411 acts on the resistances V, V to increase their respective resistance values between the wiper arm and the high side of the pots from to 2,500 ohms when the control shaft is rotated counter-clockwise looking at the outer end of this shaft. The complete range is covered in less than one turn of the shaft 40 for instance its rotation through an arc of 311% degrees. These resistance values are decreased when the control shaft is rotated clockwise. The sections 100, 102 and wiper W are so constructed as to provide a linear taper resistance characteristic of equal ohms resistance per degree of rotation of the control shaft 40. The terminals noise. In operation of the device 83 when a voltage is impressed across the input terminals Lo Hi, the device develops loop currents within the structure which develop a voltage across the output terminals. Because of the circuit configuration the input and output resistances do not vary over a wide range. Moreover when the control shaft is rotated counterclockwise there is a decrease in the amount of power delivered to the load circuit to which the device is connected and this decreasing value is registered by the indicia means 18 as increasing decibels of loss. Stated otherwise there is an increasing number of decibels of loss registered for decreasing amounts of such power. When the control shaft is rotated clockwise the result is just the reverse.

The cam drive means 16 preferably comprises a substantially rigid slide carriage and means, for instance roller means generally referred to by the numeral 111, for engaging the surface portions 42, 44 of the strip 35 and causing the rotation of the cam 34 when the carriage is moved along a fixed linear path provided by guide means such as a rod 112 whose axis parallels the axis of rotation of the cam 34. The carriage is slidably journalled on the rod 112 by bearing bushings 113 fixed in spaced upstanding wings 114 of the slide carriage. To minimize sliding friction these bushings are preferably made of synthetic plastic such as teflon. The guide rod 112 also serves to tie the end plates 28, 30 together and in abutting relation to the opposite ends of the section 20 of the casing 10. For this purpose screws 115 are secured in the ends of the rod 112. To aid in locating the end plates and lock them in position a plurality of dimples 116 are formed in the plates 28, 30 and extend inwardly at the intersecting corners of the walls of the section 20, at least two such dimples being provided on each plate. The carriage 110 includes an operating arm 117 which extends through an elongated slot 118 of the face plate of the indicia means 18 where it receives an operating knob 119 secured to it by a set screw 120. Felt washers 121 on the rod 112 provide bumpers for the carriage 110 at opposite ends of the casing 10.

The roller means 111 in the form shown in FIG. 1 and 4 preferably consists of three rollers preferably of a synthetic acetal plastic material such as Delrin one, the upper roller 122 in FIG. 4, engaging the surface portion 42 of the cam strip 35 intermediate the edges 46, 48 of the strip and two rollers 123 and 124, the lower rollers in FIG. 4, engaging the opposite surface portion 44 of the strip adjacent the edges 46, 48 and on opposite sides of the roller 122. The roller 122 is rotably carried on a metallic support or retaining rod or shaft 125 and the rollers 123 and 124 are similarly carried on a rod or shaft 126. The rods 125, 126 are mounted in a U- shaped metal bracket 127 suitably secured as by spot welding to the slide carriage 110. The rods 125, 126 extend outwardly beyond the sides 128, 130 of the bracket 127 and are spring biased toward each other by tension springs 132 the ends of which have loops 134 seated in retaining grooves (not shown) cut in the rod extensions. Suitable sleeves or compression springs 136 and end washers 138 locate the roller 122 between the sides of the bracket 127 and a sleeve or compression spring 140 and end washers 142 locate the rollers 123 and 124.

In the construction of FIG. 4 the rod 126 is preferably loosely carried in fitting bores 144 in the sides 128, 130 of the bracket 127 and the rod 125 is carried in slotted bo'res 146 so that it and the roller 122 may float vertically toward the rod 126 and under the action of the springs 132. Stated otherwise a lost motion arrangement is provided between the upper and lower rollers. It will be understood that the relationship of the upper and lower rollers may be reversed and the lost motion may be provided in the lower rod bearing bores and the upper ones be fixed. In either case the rollers 123, 124 will be biased into rolling contact with one of the surfaces 42, 44 of the cam strip and the roller 122 into rolling contact with the other. It will be noted in this connection that the slide 110 which carries the roller bracket 127 may pivot about the slide rod 112 as a pivot. This assures an effective three point geometric contact between the cam and rollers. With the tension in springs 132 sufficient to maintain roller contact with the cam strip when the carriage is moved on its guide rod 112 and to overcome any rotational resistance, the cam strip will be rotated in a clockwise direction looking at FIG. 4 when the slide is moved in the direction of the end plate 28 and counter-clockwise when the slide is moved in the direction of the end plate 30. In order to limit any rotative movement of the carriage 110 about the rod 112 if the control knob 119 is inadvertently urged counter-clockwise, as seen in FIG. 4, against the tension of springs 132 with the roller arrangement shown, the carriage is provided with a lateral extension or stop 150 which is adapted to abut the wall 26 of the casing section 20 to inhibit such action. Any tendency to rotate the carriage clockwise is limited by the cam 35 which takes the reaction through the lower rollers whose rod 126 is in fixed bores. The extension 150 is preferably tipped with a U-shaped teflon shoe 152 suitably secured thereto to provide a low friction contact between the contacting surfaces, should such occur, and thereby inhibit noise. If the roller arrangement is reversed with the floating roller shaft 125 at the bottom, the side wall 128 of the roller bracket 127 may be extended and similarly tipped as shown in phantom at 156 in FIG. 4 to provide a rotative stop against the upper wall 24 of the casing section 10.

The indicia means 18 comprises the slotted face plate 32 and manually operable control knob 119 which has a pointer-like projection or index point 160 which moves along a scale generally referred to by the numeral 162. The scale is divided into a plurality of divisions commencing with zero at the upper end and finishing with infinity at the bottom end, each division except the last representing five decibels of loss. Movement of the knob 119 and hence of the roller carriage 110 along the scale to register an increasing decibels of loss amount produces a counter-clockwise rotation of the control shaft 40 of the device 88 and as described above a reduction in the amount of power delivered to the load circuit to which the device 88 is connected. The face plate 32 is secured to the casing by hidden screws 164 which are threadedly received in the back portion of the face plate and are carried on inwardly extending ears 166, 168 respectivelyv of the end plates 28, 30 respectively. The screws are held to the ears by suitable lock washers 170 facilitating assembly of the face plate to the casing by access to the screws through suitable openings 172 in the casing. l-Ioles 174 are provided in the face plate 32 for mounting the apparatus of the invention to a console or the like. In order to shield the slide carriage and rod from dirt and the like, a shield 180 of angular form has one leg 182 secured as by screws 184 to the wall 24 of the casing and the other leg 185 forming a cover over the slide carriage and rod 112 and extending the length of the apparatus immediately below the slot 118 in the face plate 32.

FIG. 8 shows a modification of the roller arrangement of FIG. 4, being similar to that of FIG. 4 except that two rollers 190, 192 located by a spacer 140 and washers 142 replace the single upper roller 122 of FIG. 3 thus forming oppositely facing pairs 190, 123 and 192, 124 of rollers instead of the three roller geometric arrangement of rollers in FIG. 4. This modification has the advantage of providing two points of pressure application in effecting each direction of rotation of the cam but has a greater tendency to increase the sliding load on the carriage.

FIG. 9 shows a modification of the manner of providing a low friction bearing contact between the slide carriage 110 and the guide rod 112. Here the plastic bearings 113 of FIGS. 1 and 4 are omitted and the slide legs 1 14 of the carriage 110 are provided with three or four sided openings 200 (that shown being 4) to receive the rod 110.

FIGS. 10 and 11, show a resilient geometric method of slideably mounting the slide carriage 110 on the guide rod 112. Here the side legs 114 of the carriage are provided with five sided openings 210 two sides 212, 214 contacting the rod 112 at adjacent points preferably degrees apart and a further point of contact being provided midway the legs 114 by a pad 216 which may be a felt pad. The pad may be suitably secured to a flat spring member 218 that extends through the openings 210 of the legs 114 between the sides 220, 222 of the openings 210 and spaced from the bottom side 224 thereof and is tensioned by spaced adjustable screws 226 threadedly carried by the bottom wall 228 of the slide. This feature provides a low friction no lash connection between the carriage 110 and rod 112.

From the foregoing description of the invention it will be evident that a novel slide attenuator apparatus and the like has been provided in which the operating mechanism may function with a minimum of mechanical and electrical noise and without back lash or other lost motion in the mechanism likely to cause inaccurate readings. It will be understood that although the apparatus has been described in connection with varying the output characteristics of an electrical device it has many other applications. Moreover various other changes and modifications of the structure shown and described will occur to those skilled in the art without departing from the spirit and intent of the invention and all such changes, modifications and equivalent applications and structures are contemplated as come within the purview of the following claims.

What is claimed is:

1. Apparatus for operating an electrical device having a control member rotatable for varying the electrical output characteristic thereof, said apparatus comprising a substantially rigid rotatable cam characterized by a helical ribbon extending lengthwise of the axis of rotation of the cam, said ribbon having lateral edges and opposite helically extending surface portions, means for drivingly connecting said cam with said control member, cam drive means comprising roller means engaged with said opposite surface portions of said cam and movable there along for effecting rotation of said cam and thereby said control member, said roller means providing two distinctive spaced apart roller peripheries for contacting one of said opposite helically extending surface portions intermediate said lateral edges, one in juxtaposition to one of said lateral edges and the other in juxtaposition to the other of said lateral edges and at least one further roller peripheryintermediate said lateral edges for contacting the other of said opposite surface portions, spring means resiliently biasing said roller peripheries of said roller means into positive contact with said opposite surface portions of said cam ribbon such that said roller means will at all times control rotation of said cam and thereby said control member and means substantially confining movement of said cam drive means to a substantially fixed linear path.

2. Apparatus as claimed in claim 1, wherein said cam drive means comprises three roller, one engaged with one of said opposed surface portions intermediate said lateral edges of said cam ribbon and two engaged with the other of said opposite surface portions on opposite sides of said first roller.

3. Apparatus as claimed in claim 1 wherein said cam drive means comprises two pairs of opposed rollers, one pair engaged with said opposite surface portions adjacent one of said lateral edges of said cam ribbon and the other pair engaged with said opposite surface portions adjacent the other of said lateral edges of said cam ribbon.

4. Apparatus as claimed in claim 1, wherein said last mentioned means comprises a slide carriage for carrying said roller means, and a slide rod for supporting said carriage for sliding movement in a direction paralleling the axis of rotation of said cam.

5. Apparatus as claimed in claim 4, wherein the slide rod is circular in section and wherein the slide carriage has supporting bearing bores of square section receiving said slide rod.

6. Apparatus as claimed in claim 4 wherein the carriage has a geometric resilient mounting on said slide rod.

7. Apparatus as claimed in claim 1, wherein the means for drivingly connecting the cam with the control member comprises a flexible coupling.

8. A slide attenuator comprising an elongated casing having open opposite ends and an open side, end plates over said end openings and abutting said casing, a face plate over side opening and secured to said end plates, said face plate having an elongated slot therethrough extending lengthwise of said casing, a rotary potentiometer device carried by one of said end plates interiorly of said casing adjacent one end thereof, said device having a control member rotatable for varying the electrical output characteristic of the device, said control member extending in the direction of the opposite end of said casing, a substantially rigid rotatable cam extending lengthwise of said casing interiorly thereof substantially coaxial with said control member, said cam being in the form of a helically twisted ribbon presenting opposed surface portions extending helically lengthwise of the axis of rotation of the cam, one end of said cam being flexibly coupled to said control member and the other end of said cam being pivotally mounted in the other of said end plates, a slide supporting rod extending within said casing lengthwise thereof adjacent said face plate and substantially paralleling the axis of said cam, said rod having its ends secured to said end plates so as to tie said plates together in abutment to said casing, a slide carriage slidably supported on said slide rod, spring biased roller means carried by said carriage and engaged with said opposed surface portions of said cam for driving said cam whereby movement of said slide along said slide rod will cause said rollers to impart rotation to said cam and thereby to said control shaft to vary the electrical output characteristic of said potentiometer device, an operating armon said carriage extending through the slot in said face plate, a knob on said arm straddling said slot, graduations on said face plate adjacent said slot for indicating said variations in the electrical characteristics of said potentiometer device, means on said knob providing a reference point for determining said variations and a dirt shield over said slide rod intermediate said rod and said face plate and secured to said casing.

9. Apparatus for operating an electrical device having a control member rotatable for varying the electrical output characteristic thereof, said apparatus comprising a substantially rigid rotatable cam in the form of a helically twisted ribbon having lateral edges and presenting opposite surface portions extending helically lengthwise of the axis of rotation of the cam, means comprising a flexible coupling for drivingly connecting said cam with said control member, cam drive means engaged with each of said opposite surface portions of said cam ribbon and movable there along for effecting rotation of said cam and thereby said control member, said cam drive means comprising three rollers resiliently biased against said surface portions, one engaged with one of said opposite surface portions of said cam ribbon intermediate said lateral edges thereof, and two engaged with the other of said opposite surface portions of said cam ribbon on opposite sides of said first roller, and means substantially confining movement of said cam drive means to a substantially linear path.

10. Apparatus for operating an electrical device having a control member rotatable for varying the electrical output characteristic thereof, said apparatus comprising a substantially rigid rotatable cam in the form of a helically twisted ribbon having lateral edges and presenting opposite surface portions extending helically lengthwise of the axis of rotation of the cam, means for drivingly connecting said cam with said control member, cam drive means engaged with each of said opposite surface portions of said cam ribbon and movable there along for effecting rotation of said cam and thereby said control member, said cam drive means comprising three rollers, one engaged with one of said opposite surface portions of said cam ribbon intermediate said lateral edges thereof, and two engaged with the other of said opposite surface portions of said cam ribbon on opposite sides of said first roller, a slide carriage for carrying said rollers, a first shaft means supporting said one roller, a second shaft means supporting said two rollers, spring means biasing said first and second shaft means toward each other so as to provide a smooth resilient coupling between said rollers and said cam ribbon surface portions, said bias being sufficient to facilitate reversible rotation of said cam without backlash in response to operation of said slide carriage and a slide rod for supporting said carriage for sliding movement in a direction paralleling the axis of rotation of said cam.

K i 6 l t 

1. Apparatus for operating an electrical device having a control member rotatable for varying the electrical output characteristic thereof, said apparatus comprising a substantially rigid rotatable cam characterized by a helical ribbon extending lengthwise of the axis of rotation of the cam, said ribbon having lateral edges and opposite helically extending surface portions, means for drivingly connecting said cam with said control member, cam drive means comprising roller means engaged with said opposite surface portions of said cam and movable there along for effecting rotation of said cam and thereby said control member, said roller means providing two distinctive spaced apart roller peripheries for contacting one of said opposite helically extending surface portions intermediate said lateral edges, one in juxtaposition to one of said lateral edges and the other in juxtaposition to the other of said lateral edges and at least one further roller periphery intermediate said lateral edges for contacting the other of said opposite surface portions, spring means resiliently biasing said roller peripheries of said roller means into positive contact with said opposite surface portions of said cam ribbon such that said roller means will at all times control rotation of said cam and thereby said control member and means substantially confining movement of said cam drive means to a substantially fixed linear path.
 2. Apparatus as claimed in claim 1, wherein said cam drive means comprises three roller, one engaged with one of said opposed surface portions intermediate said lateral edges of said cam ribbon and two engaged with the other of said opposite surface portions on opposite sides of said first roller.
 3. Apparatus as claimed in claim 1 wherein said cam drive means comprises two pairs of opposed rollers, one pair engaged with said opposite surface portions adjacent one of said lateral edges of said cam ribbon and the other pair engaged with said opposite surface portions adjacent the other of said lateral edges of said cam ribbon.
 4. Apparatus as claimed in claim 1, wherein said last mentioned means comprises a slide carriage for carrying said roller means, and a slide rod for supporting said carriage for sliding movement in a direction paralleling the axis of rotation of said cam.
 5. Apparatus as claimed in claim 4, wherein the slide rod is circular in section and wherein the slide carriage has supporting bearing bores of square section receiving said slide rod.
 6. Apparatus as claimed in claim 4 wherein the carriage has a geometric resilient mounting on said slide rod.
 7. Apparatus as claimed in claim 1, wherein the means for drivingLy connecting the cam with the control member comprises a flexible coupling.
 8. A slide attenuator comprising an elongated casing having open opposite ends and an open side, end plates over said end openings and abutting said casing, a face plate over side opening and secured to said end plates, said face plate having an elongated slot therethrough extending lengthwise of said casing, a rotary potentiometer device carried by one of said end plates interiorly of said casing adjacent one end thereof, said device having a control member rotatable for varying the electrical output characteristic of the device, said control member extending in the direction of the opposite end of said casing, a substantially rigid rotatable cam extending lengthwise of said casing interiorly thereof substantially coaxial with said control member, said cam being in the form of a helically twisted ribbon presenting opposed surface portions extending helically lengthwise of the axis of rotation of the cam, one end of said cam being flexibly coupled to said control member and the other end of said cam being pivotally mounted in the other of said end plates, a slide supporting rod extending within said casing lengthwise thereof adjacent said face plate and substantially paralleling the axis of said cam, said rod having its ends secured to said end plates so as to tie said plates together in abutment to said casing, a slide carriage slidably supported on said slide rod, spring biased roller means carried by said carriage and engaged with said opposed surface portions of said cam for driving said cam whereby movement of said slide along said slide rod will cause said rollers to impart rotation to said cam and thereby to said control shaft to vary the electrical output characteristic of said potentiometer device, an operating arm on said carriage extending through the slot in said face plate, a knob on said arm straddling said slot, graduations on said face plate adjacent said slot for indicating said variations in the electrical characteristics of said potentiometer device, means on said knob providing a reference point for determining said variations and a dirt shield over said slide rod intermediate said rod and said face plate and secured to said casing.
 9. Apparatus for operating an electrical device having a control member rotatable for varying the electrical output characteristic thereof, said apparatus comprising a substantially rigid rotatable cam in the form of a helically twisted ribbon having lateral edges and presenting opposite surface portions extending helically lengthwise of the axis of rotation of the cam, means comprising a flexible coupling for drivingly connecting said cam with said control member, cam drive means engaged with each of said opposite surface portions of said cam ribbon and movable there along for effecting rotation of said cam and thereby said control member, said cam drive means comprising three rollers resiliently biased against said surface portions, one engaged with one of said opposite surface portions of said cam ribbon intermediate said lateral edges thereof, and two engaged with the other of said opposite surface portions of said cam ribbon on opposite sides of said first roller, and means substantially confining movement of said cam drive means to a substantially linear path.
 10. Apparatus for operating an electrical device having a control member rotatable for varying the electrical output characteristic thereof, said apparatus comprising a substantially rigid rotatable cam in the form of a helically twisted ribbon having lateral edges and presenting opposite surface portions extending helically lengthwise of the axis of rotation of the cam, means for drivingly connecting said cam with said control member, cam drive means engaged with each of said opposite surface portions of said cam ribbon and movable there along for effecting rotation of said cam and thereby said control member, said cam drive means comprising three rollers, one engaged with one of said oPposite surface portions of said cam ribbon intermediate said lateral edges thereof, and two engaged with the other of said opposite surface portions of said cam ribbon on opposite sides of said first roller, a slide carriage for carrying said rollers, a first shaft means supporting said one roller, a second shaft means supporting said two rollers, spring means biasing said first and second shaft means toward each other so as to provide a smooth resilient coupling between said rollers and said cam ribbon surface portions, said bias being sufficient to facilitate reversible rotation of said cam without backlash in response to operation of said slide carriage and a slide rod for supporting said carriage for sliding movement in a direction paralleling the axis of rotation of said cam. 